624 research outputs found
The present rate of Supernovae
We present and discuss the most recent determination of the rate of
Supernovae in the local Universe. A comparison with other results shows a
general agreement on the gross values but still significant differences on the
values of the rates of various SN rates in different kinds of galaxies. The
rate of SNe, used as a probe of Star Formation, confirms the young progenitor
scenario for SNII+Ib/c. The increasing diversity of SNe reflects also in the SN
yields which may affect the chemical evolution of the Galaxy but, because of
the limited statistics, we cannot estimate the contributions of the new
subtypes yet. It is also expected that in a few years observational
determinations of the SN rates at various look-back times will be available.Comment: 9 pages, Latex, 1 figure, To appear in the proceedings of the
conference "The Chemical Evolution of The Milky Way: Stars versus Clusters",
eds. F. Matteucci and F. Giovannelli, Vulcano, Italy, September 20-24 199
Optimal control for one-qubit quantum sensing
Quantum systems can be exquisite sensors thanks to their sensitivity to
external perturbations. This same characteristic also makes them fragile to
external noise. Quantum control can tackle the challenge of protecting quantum
sensors from environmental noise, while leaving their strong coupling to the
target field to be measured. As the compromise between these two conflicting
requirements does not always have an intuitive solution, optimal control based
on numerical search could prove very effective. Here we adapt optimal control
theory to the quantum sensing scenario, by introducing a cost function that,
unlike the usual fidelity of operation, correctly takes into account both the
unknown field to be measured and the environmental noise. We experimentally
implement this novel control paradigm using a Nitrogen Vacancy center in
diamond, finding improved sensitivity to a broad set of time varying fields.
The demonstrated robustness and efficiency of the numerical optimization, as
well as the sensitivity advantaged it bestows, will prove beneficial to many
quantum sensing applications
Collisionless Dynamics in Two-Dimensional Bosonic Gases
We study the dynamics of dilute and ultracold bosonic gases in a quasi
two-dimensional (2D) configuration and in the collisionless regime. We adopt
the 2D Landau-Vlasov equation to describe a three-dimensional gas under very
strong harmonic confinement along one direction. We use this effective equation
to investigate the speed of sound in quasi 2D bosonic gases, i.e. the sound
propagation around a Bose-Einstein distribution in collisionless 2D gases. We
derive coupled algebraic equations for the real and imaginary parts of the
sound velocity, which are then solved taking also into account the equation of
state of the 2D bosonic system. Above the Berezinskii-Kosterlitz-Thouless
critical temperature we find that there is rapid growth of the imaginary
component of the sound velocity which implies a strong Landau damping. Quite
remarkably, our theoretical results are in good agreement with very recent
experimental data obtained with a uniform 2D Bose gas of Rb atoms.Comment: 5 pages, 2 figures, improved introduction and conclusions, accepted
for publication in Physical Review
Quantum sensing
"Quantum sensing" describes the use of a quantum system, quantum properties
or quantum phenomena to perform a measurement of a physical quantity.
Historical examples of quantum sensors include magnetometers based on
superconducting quantum interference devices and atomic vapors, or atomic
clocks. More recently, quantum sensing has become a distinct and rapidly
growing branch of research within the area of quantum science and technology,
with the most common platforms being spin qubits, trapped ions and flux qubits.
The field is expected to provide new opportunities - especially with regard to
high sensitivity and precision - in applied physics and other areas of science.
In this review, we provide an introduction to the basic principles, methods and
concepts of quantum sensing from the viewpoint of the interested
experimentalist.Comment: 45 pages, 13 figures. Submitted to Rev. Mod. Phy
Nonequilibrium Kinetics of One-Dimensional Bose Gases
We study cold dilute gases made of bosonic atoms, showing that in the
mean-field one-dimensional regime they support stable out-of-equilibrium
states. Starting from the 3D Boltzmann-Vlasov equation with contact
interaction, we derive an effective 1D Landau-Vlasov equation under the
condition of a strong transverse harmonic confinement. We investigate the
existence of out-of-equilibrium states, obtaining stability criteria similar to
those of classical plasmas.Comment: 16 pages, 6 figures, accepted for publication in Journal of
Statistical Mechanics: Theory and Experimen
Equation of state and self-bound droplet in Rabi-coupled Bose mixtures
Laser induced transitions between internal states of atoms have been playing
a fundamental role to manipulate atomic clouds for many decades. In absence of
interactions each atom behaves independently and their coherent quantum
dynamics is described by the Rabi model. Since the experimental observation of
Bose condensation in dilute gases, static and dynamical properties of
multicomponent quantum gases have been extensively investigated. Moreover, at
very low temperatures quantum fluctuations crucially affect the equation of
state of many-body systems. Here we study the effects of quantum fluctuations
on a Rabi-coupled two-component Bose gas of interacting alkali atoms. The
divergent zero-point energy of gapless and gapped elementary excitations of the
uniform system is properly regularized obtaining a meaningful analytical
expression for the beyond-mean-field equation of state. In the case of
attractive inter-particle interaction we show that the quantum pressure arising
from Gaussian fluctuations can prevent the collapse of the mixture with the
creation of a self-bound droplet. We characterize the droplet phase and
discover an energetic instability above a critical Rabi frequency provoking the
evaporation of the droplet. Finally, we suggest an experiment to observe such
quantum droplets using Rabi-coupled internal states of K atoms.Comment: to be published in Scientific Report
The 3-D ionization structure and evolution of NGC 7009 (Saturn Nebula)
Tomographic and 3-D analyses for extended, emission-line objects are applied
to long-slit ESO NTT + EMMI high-resolution spectra of the intriguing planetary
nebula NGC 7009, covered at twelve position angles. We derive the gas expansion
law, the diagnostics and ionic radial profiles, the distance and the central
star parameters, the nebular photo-ionization model and the spatial recovery of
the plasma structure and evolution. The Saturn Nebula (distance~1.4 kpc,
age~6000 yr, ionized mass~0.18 Mo) consists of several interconnected
components, characterized by different morphology, physical conditions,
excitation and kinematics. The internal shell, the main shell, the streams and
the ansae expand at V(exp)~4.0xR" km/s, the outer shell, the caps and the
equatorial pseudo-ring at V(exp)~3.15xR" km/s, and the halo at V(exp)~10 km/s.
We compare the radial distribution of the physical conditions and the line
fluxes observed in the eight sub-systems with the theoretical profiles coming
from the photo-ionization code CLOUDY, inferring that all the spectral
characteristics of NGC 7009 are explainable in terms of photo-ionization by the
central star, a hot (logT*~4.95) and luminous (log L*/Lo~3.70) 0.60--0.61 Mo
post--AGB star in the hydrogen-shell nuclear burning phase. The 3--D shaping of
the Saturn Nebula is discussed within an evolutionary scenario dominated by
photo-ionization and supported by the fast stellar wind: it begins with the
superwind ejection, passes through the neutral, transition phase (lasting ~
3000 yr), the ionization start (occurred ~2000 yr ago), and the full ionization
of the main shell (~1000 yr ago), at last reaching the present days: the whole
nebula is optically thin to the UV stellar flux, except the caps and the ansae.Comment: accepted for pub. in A&A, 28 pages, 14 figures, full text with
figures available at http://web.pd.astro.it/supern/ps/h4665.ps, movies on the
3D structure available at http://web.pd.astro.it/sabbadin
SN/GRB connection: a statistical approach with BATSE and Asiago Catalogues
Recent observations suggest that some types of GRB are physically connected
with SNe of type Ib/c. However, it has been pointed out by several authors that
some GRBs could be associated also with other types of core-collapse SNe (type
IIdw/IIn). On the basis of a comphrensive statistical study, which has made use
of the BATSE and Asiago catalogues, we have found that: i) the temporal and
spacial distribution of SNe-Ib/c is marginally correlated with that of the
BATSE GRBs; ii) we do not confirm the existence of an association between GRBs
and SNe-IIdw/IIn.Comment: Proceeding of the 4th workshop on Gamma Ray Bursts in the Afterglow
Era, Rome, 2004; 4 page
Measurement of the excited-state transverse hyperfine coupling in NV centers via dynamic nuclear polarization
Precise knowledge of a quantum system's Hamiltonian is a critical pre-requisite for its use in many quantum information technologies. Here, we report a method for the precise characterization of the nonsecular part of the excited-state Hamiltonian of an electronic-nuclear spin system in diamond. The method relies on the investigation of the dynamic nuclear polarization mediated by the electronic spin, which is currently exploited as a primary tool for initializing nuclear qubits and performing enhanced nuclear magnetic resonance. By measuring the temporal evolution of the population of the ground-state hyperfine levels of a nitrogen-vacancy center, we obtain the first direct estimation of the excited-state transverse hyperfine coupling between its electronic and nitrogen nuclear spin. Our method could also be applied to other electron-nuclear spin systems, such as those related to defects in silicon carbide
- …